This invention generally pertains to a system for pre-amplifying signals detected by the receive or reception coil of a magnetic resonance (MR) system and, more particularly, to such system wherein the preamplification circuitry is optically isolated from other system electronics, hereinafter referred to collectively as MR signal processing electronics, which receives and further processes pre-amplified MR signals from the preamplification circuitry.
In an MR system, whether designed for spectroscopy, imaging or other application, an RF pulse is transmitted into a subject of interest (e.g., body tissue). In response, the subject emits a signal which is detected by an MR receive coil and thereafter processed to obtain information about the subject.
MR signals detected by the receive coil are inherently very weak and therefore require preamplification before further signal processing can take place. Common practice is to connect a preamplifier to the MR receive coil by means of a relatively long electric cable. Preamplifier components typically include a first impedance matching network connected between the receive coil and the cable input, a remotely located active preamplifier device, such as a GaAs-MESFET (gallium arsenide metal-semiconductor field-effect transistor) or HEMT (high electron mobility transistor), and a second impedance matching network connecting the cable output to the active device input. The second matching network is required to match the output impedance of the cable, which is on the order of 50 ohms, to the input impedance of a device such as a GaAs-MESFET, which is on the order of 1200 ohms, for optimal noise performance.
Noise in the cable and the respective components of the preamplifier may be very serious, in view of the comparative weakness of received MR signals. If the level of noise collectively resulting from these noise sources is too great, the preamplifier will not be able to achieve a signal-to-noise ratio that is high enough to enable the signal processing electronics to further process the received MR signals. Even more noise may be present if the system uses multiple receive coils, each of which is separately coupled to a respective preamplifier through a respective electric cable. For example, in a phased array configuration, numerous signal receive coils are positioned together, and each must be precisely tuned. Maintaining proper tuning of each coil is very difficult with the conventional arrangement of one electric cable for each coil.